Recently I saw two seagulls stomping on grass.
Is this a mating ritual or is it done to force food to the surface?

John Warham, a zoologist at the University of Canterbury, responded.

Gull "stomping" such as you describe has been reported of several kinds
of gulls in different parts of the world and does seem, as you suggest, to
be to drive food to the surface.

Shorebirds may also do this. Along the tide-line their prey seems
to be small invertebrates such as amphipods and on grass-lands worms,
grass-grub lavae and such like. Other wading birds, such as stilts
and herons, feeding in the shallows are sometimes seen stirring the
mud with their feet, again to disturb hidden prey.

Why vibrations should cause animals to emerge from the grass or
to reveal themselves to predators is obscure. You would think
they would take alarm and dig down to safety.